1. Field of Technology
The invention relates to a method for the continuous preparation of a cellulose suspension in an aqueous tertiary amine oxide for use in the Lyocell method. The invention also relates to a device for performing said method.
2. Description of Related Art
It is known to prepare cellulose form- and spinning masses from cellulose solutions in amine oxides, preferably in N-methylmorpholine-N-oxide and a non-soluble agent for cellulose, preferably water. By forming the solution and drawing and regenerating the cellulose, products are obtained for diverse use in the textile- and non-textile area (see W. Berger, Options and Limits of Alternative Cellulose Dissolution and -Drawing, Lanzinger Reports 74 (1994) 9, pages 11 to 18). BISFA assigned the generic term “Lyocell” to fibers produced with this method.
WO 94/28217 discloses a discontinuous method for the preparation of a cellulose suspension in aqueous amine oxide, where comminuted cellulose and an amine oxide solution are mixed by a rotor with radial stirring elements in a horizontal mixing chamber. The duration per batch is stated as 21 minutes. This method of operation is disadvantageous because two such mixing chambers have to be operated in parallel due to the continuous load of the subsequent dissolution step. Furthermore, the complete emptying of the mixing chambers leads to difficulties.
It is also known from WO 96/33221 to prepare a cellulose suspension in aqueous N-methylmorpholine-N-oxide (NMMO) by mixing the comminuted cellulose in an annular layer mixer directly with the aqueous NMMO, such as 75 percent-by-mass NMMO. The formed suspension is brought to dissolution in a separate film extruder. The disadvantage of the annular layer mixer is that only comminuted, essentially dry cellulose can be used. If the cellulose contains water, the formation of layers in the mixer is difficult due to the mixing with NMMO solution, which was added separately. The water has to be separated thermally. The soluble components of the cellulose reach the spinning solution and lead to disadvantageous properties in the cellulose products. Because the suspension is transported as a layer, the throughput relative to the cross-section of the apparatus is low.
From DE 198 37 210.8, it is known to suspend the cellulose in water prior to the formation of a suspension in amine oxide solution, and to partially separate it again from the suspension agent after a certain amount of time. Then the wet cellulose material is first conveyed through a horizontal shear zone in the absence of amine oxide, and then together with amine oxide. This method of operation requires a relatively long shear zone and thus a significant effort with respect to apparatus. The formation of a uniform suspension is complicated by the fact that the aqueous oxide is fed into a cellulose material that completely fills the available volume of the apparatus.
The present invention is based on the problem to provide a method and a device for the continuous preparation of a cellulose suspension in an aqueous tertiary amine oxide with a reduced apparatus effort for the formation of suspensions and the activation of cellulose for use in the Lyocell process. Especially, the throughput of the cellulose material relative to the volume of the shear zone is supposed to be increased, and thus the preparation time of the suspension is supposed to be reduced. Other advantages are shown in the following description.
Thus, the invention is based on a method for the continuous preparation of a cellulose suspension in an aqueous tertiary amine oxide for use in the Lyocell process, where (a) a cellulose suspension is formed from cellulose and an amine oxide-free phase in a mass ratio in the range of 1:3 to 1:40, (b) the cellulose suspension is dehydrated into a material with a cellulose content in the range of 20 to 80 percent-by-mass, and (c) the wet cellulose material is mixed with enough aqueous amine oxide and conveyed through a horizontal shear zone so that after the mixing, a suspension with an amine oxide content in the liquid phase in the range of 70 to 80 percent-by-mass is obtained, whereby the suspension essentially completely fills the available conveying diameter in the shear zone.
In accordance with the invention, this method is characterized in that in step (c), aqueous amine oxide in finely dispersed form is added to the wet cellulose material in a fall zone only partially filled with the cellulose material, and the cellulose material mixed with the amine oxide is introduced into the shear zone. In a variant of the method according to DE 198 37 210.8, the first part of the shear zone, where the cellulose material is conveyed and homogenized in the absence of NMMO, is obsolete. Furthermore, the aqueous amine oxide is not introduced into a shear zone that is essentially completely filled with the suspension, but rather only into the upstream fall zone. The cellulose material fills the fall zone only partially, so that the introduction of the amine oxide simultaneously leads to a pre-distribution of the amine oxide in the wet cellulose material. It is said pre-distribution of the amine oxide that allows a shortening of the shear zone for the formation of a uniform suspension for the subsequent formation of the solution, which follows the fall zone. Compared to the method of operation according to DE 198 37 210.8, this results in a shortening of the shear zone by approximately one third, and a reduction of investment costs by approximately 10 to 15 percent.
According to the preferred embodiment of the method in accordance with the invention, the aqueous amine oxide is injected into the fall zone. In that way, the falling particles of the wet cellulose material are enveloped by the aqueous amine oxide and a good pre-mix is obtained, which allows a shortening of the subsequent homogenization frame in the shear apparatus. Preferably, N-methylmorpholine-N-oxide-monohydrate is injected into the fall zone.
Surprisingly, it was found that the quality of the pulp was improved, despite the reduction of the process steps and/or the production time of the suspension, which can be determined by means of the very good swelling condition of the cellulose particles in the three-substance-mixture cellulose/NMMO/water at the end of the shear zone. This is attributed to the fact that the fine dispersion of the amine oxide in the fall zone shortens the time period otherwise required until the NMMO solution acts on the cellulose particles, so that the overall swelling of the particles has already progressed farther at the end of the shear zone. The quantity of the aqueous amine oxide added in the fall zone is coordinated with the starter cellulose quantity such that the amine oxide content in the aqueous phase of the formed suspension is in the target range of 70 to 80 percent-by-mass.